1. Field
The present invention relates generally to the field of telecommunications.
2. Background
It relates more particularly to the processing of a request conforming to the Session Initiation Protocol (SIP protocol), sent over a telecommunication network, and passing through a plurality of nodes of the network to a recipient, one of these nodes having detected an event obstructing it from processing the request and therefore from transmitting it to the next node.
In the sense of the invention, a node of an SIP network designates any entity of this network capable of routing an SIP request (i.e. conforming to the SIP protocol) to its destination. Such a node will preferably be a physical network element.
While the present invention is described herein in connection with certain embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.
Thus, the invention has a preferred, but not limiting, application in the context of voice-over-IP (VoIP) networks based on the SIP protocol or, for example, with an IMS (Internet Multimedia Subsystem) network.
An event or a situation obstructing a node of the network from processing an SIP request designates, for example, a lack of resources on that node, a congestion or a break of its links to the downstream nodes of the network on the path identified for the request, etc. Hereinafter in the description, the terms downstream and upstream will be used with reference to the path taken by the request, in the request source to request destination direction.
Now, it may be that there are other paths in the network for this same request, that is to say, paths which arrive at the same recipient but pass through different nodes. Consequently, it may be useful for a node upstream of the node having detected the obstruction (hereinafter designated “node down”) to try an alternative path: this is referred to as rerouting of the request.
In the current SIP networks, when an intermediate node A0, present on the path of an SIP call request, is in a state of temporary congestion or maintenance, it may be configured so as to send to the node A1, preceding it on the path, a failure response 503 “Service Unavailable”. This message is defined in more detail in the document RFC 3261 entitled “SIP: Session Initiation Protocol”, published by the IETF.
According to the SIP protocol, upon the reception of a failure response 503, the node A1 must try to reroute the SIP call request to a node other than the node A0. If the node A1 does not find on its level other nodes enabling the SIP call request to be routed to its recipient (either because there are none, or because said nodes are also down), a rerouting of the request is then undertaken by a node A2 located upstream of the node A1, then a node A3, etc.
Now, it may be that there is no rerouting that allows the call request to be routed to its recipient, regardless of the level at which it is undertaken in the path. This situation is called ineffective rerouting.
There may be many such ineffective reroutings depending on the topology of the network which are, because there are many, detrimental to the correct operation of the telecommunication network. They are in fact the cause of an unnecessary network load, which may be considerable and costly to other calls.
For example, with reference to FIG. 1, in a configuration in which an SIP call request sent by a calling party (CALLER) has to pass through four nodes to reach its recipient (CALLEE) and the fourth node N4 sends a failure response 503 “Service Unavailable”, the preceding nodes are each in turn, on reception of this failure response, likely to try an ineffective rerouting of the request. In the example represented in FIG. 1, this gives rise to four successive sendings of the request (transitions 4, 8, 14, 18), which result in four failure responses (transitions 5, 9, 15, 19) sent by the fourth node N4 before the failure of the call is notified to the calling party (transition 22). In other words, the network load resulting from a single SIP request, ultimately ineffective in this configuration, is multiplied by four on the fourth node.
It will therefore be clearly understood that the multiple rerouting attempts undertaken in the SIP network can not only substantially increase the unnecessary load in the network but also the congestion of the nodes already down.
Moreover, the SIP protocol, in its current version, allows the rerouting of a call request only in very limited cases of obstruction of the node from processing the call request, and following the sending of a failure response 503. Now, notably because of the multiplication of the ineffective reroutings that can be encountered in the context of the use of this failure response, said failure response is ultimately little used by the telecommunication network operators.